Need some suspension advice

[Paul Robertson]

I recently bought the Qa1 ball joints recommended in the WIKI page for SLC's that will see a lot of track use. I have 18" CCW forged track wheels that barely clear the rear ball joints and control arm with the old stock ball joints. The stock ball joints mount underneath the lower control arm. The new ball joints are longer on the bottom (because they have adjustable preload) and as a result my wheels will not clear them. I can mount them on top of the control arm which results in the controller arm being defected down 7/16".
SO, my question is how sensitive is the rear suspension geometry? Will it be deleteriously affected because the rear upright is essentially 7/16" taller (and) the separation between the upper arm and the lower arm increased by 7/16". Does anyone have any experience with these ball joints and how did you mount them in the rear (front is not a problem)?

 

[Terry Oxandale]

Don't have an SLC, but it appears you may be changing the outer pivot point on the uprights, which will change camber gain.

If the ball joint is going into the receiver just like the original one was, and it is longer than the original one (e.g. the distance from the ball center to say, the bottom plate of the upright), then the geometry is being changed anyway, and positioning the arm above or below the fixed-position ball joint shouldn't make any difference because the pivot point of the ball joint is what determines the geometry you've referenced.

 

Hi Paul.,
I'd check with RCR who should be able to advise on the change and will have the geometry drawn out. They might even have a dynamic computer model that can accurately give you the suspension data.
Cheers Roger Allen

 

[Will Campbell]

The 18" rear CCWs are known to have very tight clearances at the barrel/ball joint interface, even with the stock ball joints. Most cars with 19 or 20 inch diameter rear wheels will have adequate clearance even with ball joints that are a bit oversize.

In your case, perhaps careful grinding of the ball joint would regain the clearance you need. Luckily, the CCWs are among the most rigid of wheels, so even a small clearance may be sufficient under load.

Don't move the joint up as this will definitely affect the geometry in a bad way.

 

[Scott]

Beyond geometry issues, I think you might create a potential structural issue. It seems to me that the vast majority of the force attempts to push the ball joint upwards – i.e., hitting a bump results in a lot more force than when the suspension droops. Mounting the ball joint under the A arm means that the upward force is distributed across the ball joint’s entire flange whereas mounting it on top means that that force is distributed across four small bolts sitting on the aluminum control arm.

If you can't grind it as pnut suggested, you might consider one of the other ball joints listed on the wiki. I know that Ken is a big fan of the Moogs.

 

[Paul Robertson]

Scott, I hadn't thought of that and it makes good sense to me. Also, contemplating Will's
thoughts about the tight clearance if I realized that If I put the Ball joint above the arm there is no way my 18" wheels will clear as it will increase the distance between the axle centerline and the bottom of the arm. Duh!
Thanks guys.